1. Field of the Invention
The present invention relates to a device for detecting air-fuel ratio of an engine in which an oxygen sensor of oxygen-concentration proportional type is used.
2. Prior Art
A prior art system of this kind is shown in FIG. 4. Use is made of an oxygen sensor of the oxygen-concentration proportional type which includes oxygen-ion conductive and solid electrolyte materials 1 and 2, a spacer 3 made of inorganic heat-resistant adhesive material interposed between said electrolyte materials to partly form restricted region, such as a gap G, therebetween and a pair of opposing electrodes 4 and 5 arranged at the positions of said gap G, respectively, one of which constitutes an oxygen pump element 6 and the other of which constitutes a cell element 7 for measuring an oxygen concentration ratio. The air-fuel ratio of the engine is detected by measuring the oxygen concentration in the exhaust gas, for example.
Such oxygen sensor is positioned in the exhaust gas pipe and a D.C. voltage V.sub.B is applied between the electrodes 4 at the side of the oxygen pump element 6, with the polarity as shown in the drawing. Then, the oxygen ion passes through the solid electrolyte material 1 of the oxygen pump element 6 and the oxygen in the gap G leaks through said material to the outside. Accordingly, a difference in oxygen concentration between the gap G and the outside is produced so that an electromotive force E is generated by the cell element 7. At this time the electromotive force E produced by the cell element 7 is compared with a reference voltage by means of a comparator CMP and the pump current Ip fed to the oxygen pump element 6 is subjected to feed-back control so that E=Vs. Thus the oxygen concentration in the exhaust gas is obtained, depending upon the value of the pump current Ip which is obtained at this moment with a linear characteristic.
The output signal of the oxygen sensor produced at this time is proportional to the air-fuel ratio of the engine. For example, it is assumed that the reference voltage Vs is set at 40 mV. Then, if the variation of value of the output voltage converted from the value of the pump current Ip is within the range of 0.about.1.5 V, it is possible to detect the air-fuel ratio of the engine within the range of 14.6.about.27 air-fuel ratio.
However, a problem resides in the case of detection of the air-fuel ratio of the engine by means of the oxygen sensor arranged in the exhaust pipe. That is, pulsations arise in the exhaust gas, depending upon the operation of the engine and thus a hunting arises in the output signal of the oxygen sensor (see FIG. 2(a)), so that an error may be produced in the value of the air-fuel ratio of the engine detected on the basis of the output signal of the oxygen sensor. Particularly, in the case of a lean-burn engine, when the air-fuel ratio control is effected to hold the air-fuel ratio of the engine at a predetermined value, in accordance with the output signal of the oxygen sensor, the accuracy of the air-fuel ratio control is considerably reduced, owing to the error of the air-fuel ratio detected by the oxygen sensor having a linear characteristic in the lean area.